SockSend: remove extraneous msg_controllen assignment + micro nit fixups.

My understanding is that:
CMSG_SPACE macro accounts for the possible trailing padding in the case
of multiple ancillary messages (cmsghdr + cmsg_data) within the same msghdr.
And msghdr.msg_controllen is the sum of individual iteration step sizes -
i.e. sum of CMSG_SPACE of each contained ancillary message.

To clarify, as the current code is only sending a single ancillary message,
the "wrong" length given to the outer msg_controllen is fine in practice,
but I think it's still cleaner to use the correct (general case) macro.

Extra refs:
* kernel macros:
  https://elixir.bootlin.com/linux/v4.19/source/include/linux/socket.h#L97
* ascii diagram from elsewhere in the kernel:
  https://elixir.bootlin.com/linux/v4.19/source/net/sctp/socket.c#L8082

Change-Id: I8c66d8cb7b412e4b53fe83b41ca7bba9b341d739